Can anyone point me to a Forth implementation where the absolute bare minimum is written in (C/assembly/whatever) and everything possible is then bootstrapped in Forth itself?
I recall reading what the minimal word set needed to be able to write the rest of standard Forth (Fig Forth if I recall) but I seem to remember that most implementations don't push the purity quite that far for performance reasons.
zForth is yet another Forth, but with some special features not found in most other forths. Note that zForth was written for engineers, not for language purists or Forth aficionados. Its main intention is to be a lightweight scripting language for extending embedded applications on small microprocessors. It is not particularly fast, but should be easy to integrate on any platform with a few kB's of ROM and RAM.
I did exactly this, was curious if I could write an outer and inner interpreter in pure c in under 500 lines with only 5 c functions. I then used that to boot strap an image that can run using only the inner interpreter.
In addition to what others have linked, look up "moving forth," which is a series on porting forth... It covers the low-level bits that you'd need to port to run Forth on a new platform.
I recall reading what the minimal word set needed to be able to write the rest of standard Forth (Fig Forth if I recall) but I seem to remember that most implementations don't push the purity quite that far for performance reasons.